The present invention relates to a method for defoaming gas purification scrubbing liquids such as are commonly used in the purification of natural gas.
Foaming of the stripping or scrubbing liquid is a problem encountered in various gas purification, fractionation, stripping and similar operations. For example, in a typical natural gas purification system, the gas is passed counter-currently through a scrubbing solution, typically an aqueous ethanolamine solution, in an absorption column wherein hydrogen sulfide and other contaminants are removed from the gas by the scrubbing solution. The hydrogen sulfide and other contaminants are then separated from the scrubbing solution by a stripper still and a sock filter to thereby regenerate the scrubbing solution so that it can be again used to scrub the natural gas. The scrubbing solution is thus used in a cyclical manner. Unfortunately, in many cases contaminants build up in the scrubbing solution and cause the solution to foam.
Foaming of the scrubbing solution is very disadvantageous. Efficient scrubbing of the natural gas by the scrubbing liquid is dependent upon the gas phase being in contact with the liquid phase and foaming of the liquid phase results in reduced contact therebetween. Also, foam in the upper portion of the stripper still increases the pressure drop across the inlet and outlet streams of the scrubbing solution into and out of the still. This results in low hydraulic gradient, downflow flooding, weeping, entrainment flooding, and lower overall column efficiency as well as reduced absorption and contacting of gas and liquid phases. Furthermore, heavy foaming results in loss of scrubbing solution through the top of the stripper still and can quickly result in shutdown of the operating unit and consequent loss of production.
Foaming is enhanced by the presence of heat and low pressure in the stripper still. Unfortunately, the emulsions and foam-forming compounds built up in the scrubbing solution can provide very stable foams and although various defoaming agents such as vegetable and mineral oils and silicon compounds can be added to the solution in an attempt to control foaming, the amount of foaming encountered in some gas purification systems is so heavy that such defoaming compounds are not effective.
Wherefore, it is an object of the present invention to provide a method for controlling foaming of a gas scrubbing solution used in gas purification systems. The method of this invention relates to a gas purification system wherein an aqueous amine scrubbing solution is employed to remove contaminants such as hydrogen sulfide from the gas and then the scrubbing solution is regenerated by separating the contaminants therefrom so that the scrubbing solution can be employed in a cyclical fashion. In accordance with the method of the present invention, after the stripper still, the main stream of scrubbing solution is divided into a major stream and a minor stream; the major stream being passed through a conventional sock filter while the minor stream is passed through a side stream filter of activated charcoal.